1. Field of the Invention
The present invention relates to filtering apparatus for a reflow oven and a system for reflow soldering.
2. Description of the Related Art
Reflow soldering is now an established method of soldering electronic components to printed circuit boards. The printed circuit boards are produced using a screen printing process in which solder paste containing solder, flux, adhesives and binders is applied to a board in a required configuration. Components are then glued to the printed circuit board at appropriate locations by a pick and place machine. The components may be applied to just one side of the board or, in other cases, both sides.
The reflow soldering process typically takes place by passing a printed circuit board, with its components attached, along the conveyor belt through the tube (or tunnel) of a reflow oven. Within the oven, the board passes through a high temperature region, typically in excess of 200 degrees centigrade, in which the solder melts and forms a joint between the circuit of the board and the respective components. Flux within the paste reacts with metallic surfaces to remove oxide and enhance wetting. The soldering process takes place at a high temperature, but if the temperature is too high damage is caused to, at least, the more sensitive electronic components. Thus, the temperature must be carefully controlled within pre-defined limits, the limits themselves depending on the particular components, solder paste, etc. that are being used.
After the solder joints are formed, the conveyor passes the board through a cooling region of the oven, in which the solder solidifies, before it emerges from the oven.
The oven typically contains a process gas atmosphere of nitrogen, so that oxidation of the board and components is minimized during heating. Consequently, fresh nitrogen is continually input into the oven at considerable expense.
Heat within the oven vaporizes unreacted flux, binders and adhesives contained within the solder paste, while other vapors are liberated by the reaction of the flux on the oxidized contacts of components. Thus, the nitrogen atmosphere becomes contaminated by the aforesaid vapors.
If the contaminated nitrogen migrates into the cooling region or cooler regions of the oven, at least some of the vapors will condense on the cooler surfaces and may drip onto the circuit boards, thus producing defects. Consequently, it is known to have filtering apparatus which extracts contaminated process gas from the oven, filters it to remove most of the flux vapors, and re-inputs the filtered gas into the oven. However, contaminated gas is replaced by filtered gas, new cool nitrogen is continually input to the oven, and the new cool nitrogen expands on heating; therefore the net result is that contaminated process gas tends to be pushed out from the ends of the oven's tunnel. Since the contaminated gas must pass through cooler regions of the oven on its way to the ends of the tunnel, the aforementioned condensation occurs.
In addition, in known systems, the contaminated gas escaping from the ends of the tunnel is sucked away to be expelled to the open air, possibly via a filtration unit. The expelled gas is typically passed through ducting out into the air above the roof of the working area. Such a system has several disadvantages as a consequence. Firstly, since the ducting exhausts the gas above the roof, the variability of wind speed can cause variations in the oven's operation, in particular, its working temperatures. Secondly, if for any reason the oven is required to be relocated, the cost of such is increased due to the need to provide ducting through the roof. Thirdly, providing suction at the ends of the oven's tunnel tends to increase the flow of contaminated gas along the tunnel and enhance the previously discussed condensation problem.